Wireless telecommunication systems, such as the well-known cellular and cordless telephone and data transmission systems, typically comprise a plurality of mobile or portable radio communication units and a plurality of radio access units. Each access unit provides a number of radio communication channels to a geographical area or cell defined by the operating ranges of the access unit. The access units are connected to a central interface unit, also called radio exchange (RE) or mobile telephone switching office (MTSO). The RE or MTSO are in turn coupled to a Public Switch Telephone Network (PSTN) or Integrated Services Digital Network (ISDN), in some cases via an intermediate Private (Automatic) Branch Exchange (P(A)BX), for completing telephone and data calls between mobile radio and landline subscribers.
Typical cellular communication systems provide coverage over a relatively wide area, i.e. relatively large cells. First generation cellular mobile networks provide service to macrocells having a range from the radio access unit to the cell boundary of 1 to 5 km, and large cells (5 to 35 km). Analogue cellular systems, such as designated AMPS, ETACS, NMT-450 and NMT-900 have been deployed throughout the world. Digital cellular systems are designated IS-54B in North America and the pan-European GSM system. These systems and others are described, for example, in the book titled "Cellular Radio Systems", by Balston et al., published by Artech House, Norwood, Mass., 1993.
Cordless radio communication systems, ranging from simple residential cordless telephones to business cordless communication systems capable of serving hundreds or even thousands of cordless radio communication units across (large) offices, production halls etc., have been developed for use in picocell (a few meters), nanocell (up to 10 m) and microcell (10 to 400 m) applications. Analogue cordless telephones are designated CT0, CT1 and CT1+. Amongst the digital cordless systems, designated CT2, CT2-CAI, CT3 and DECT (Digital Enhanced Cordless Telecommunications), both CT3 and DECT use TDMA (Time Division Multiple Access) as their transmission technique, whereas CT2 operates under FDMA (Frequency Division Multiple Access). In particular in North America, spread spectrum access is used for cordless radio communication. CDMA (Code Division Multiple Access) is another digital access technique which can be used for cordless communication. Reference is made to a paper by C. Buckingham et al., "A Business Cordless PABX Telephone System on 800 MHz Based on the DECT Technology", IEEE Communications Magazine, 29(1991) January, p. 105-110.
A further type of wireless communication system is called Radio in the Local Loop (RLL). RLL provides a radio link to complete the final connection between residential subscribers and the local exchange of a PSTN/ISDN, for example.
Within the concept of RLL, two basic systems can be distinguished: Fixed RLL (FRLL) and Mobile RLL (MRLL). In a telephony FRLL system, for example, the subscriber is provided with an ordinary telephone socket, however connected to a radio transceiver also called Fixed Access Unit (FAU) or Wireless Fixed Access Unit (WFAU). Via this FAU/WFAU a radio link is established with a radio access unit, which in turn is coupled to a central interface unit providing access to the PSTN/ISDN. In the MRLL concept, the subscriber is provided with a portable cordless or mobile radio telephone handset by which, via the radio access unit, access to the PSTN/ISDN can be established.
Mixed concepts are also possible, i.e. FRLL providing mobility in the subscriber premises, also called Cordless In The Home (CITH) and residential or neighbourhood mobility, also called Cordless In The Neighbourhood (CITN).
The radio access units in an RLL system may provide service to remote communication units in their respective coverage area, i.e. pico-, nano-, micro- or macrocell, dependent on the type of radio access unit used.
Different from landline connections, a call from a communication unit in a wireless radio communication system often has to change its radio communication link due to degrading link quality. This, for example, because another communication unit having a call in progress on the same radio link moves into the coverage of a particular communication unit, or if the communication unit itself moves out of the coverage area of the radio access unit to which it currently connects. The action of switching a call in progress from one or more physical radio links or channels to other physical radio links or channels is called handover. Such a handover can be solely completed within the radio access unit to which a radio communication unit is currently connected, a so-called "intra-cell handover". In case a call is continued via a radio access unit serving another cell of a particular radio communication system, this type of handover is called "Inter-cell handover". A call handover to another radio communication system is called "inter-system handover" or "external handover".
In the idle or standby mode of a radio communication unit, i.e. when no call is in progress, and the radio communication is moving, for example, location updates and other information and activities are performed by the radio communication unit, which process is called "roaming". Roaming within the same system is called "intra-system roaming" and roaming to another communication system is called "inter-system roaming" or "external roaming".
In a radio communication system operating in accordance with the DECT standard, for example, handover or roaming can be initiated by evaluating the received radio links with regard to the transmission quality of the radio links, using either one or a combination of the following criteria: RF signal level (RSSI), burst synchronisation (SYNC) error, system information field test word (A CRC) error, data field test word (X CRC) error. Besides transmission criteria, other parameters such as system identification, access rights etc. may be included in the decision of initiating handover or roaming.
In a radio environment in which all the radio access units are operated in a time synchronous manner, inter-cell and inter-system handover and roaming can be performed without timing reference constraints using one or a plurality of the above criteria. However, in large radio communication networks and in the case of access units connected to different networks or systems, a time synchronous operation of all the access units can be difficult to achieve or only with a high and costly effort.
To perform inter-cell and inter-system handover in an asynchronously operating radio environment, in order to transfer a call from a first radio access unit to a second radio access unit, adjustment of the timing reference of the radio communication unit is required in order to control the single transceiver devices of the radio communication unit to communicate with both the first and second radio access units during a certain period of time, i.e. during the call transfer phase.
Swiss patent CH 0,682,867 and British patent application GB-A-2,281,117 disclose methods for performing handover in a mobile radio telecommunication system, wherein a call without interruption of the ongoing service is transferred from a first radio link to a second radio link. A handover which does not cause a degradation of the service provided is called a "seamless handover".
To execute a seamless handover, during a certain period of time two duplex radio links between a radio access unit and a communication unit have to be maintained simultaneously. In the case of a handover request, the call at the first radio link is maintained while a second radio link is established. Only if data over the second radio link is successfully exchanged in both directions, the first radio link is terminated.
However, the majority of transceiver control devices used in current radio communication units are provided with a single timing reference to operate in time-synchronous radio networks. Due to lack of a second timing reference the transceiver control devices are not able to simultaneously support two or even more asynchronously operated radio links, as required for a seamless handover in an asynchronously operating radio environment.
In an asynchronously operating radio environment, handover by a radio communication unit having a single timing reference is effected by first releasing the existing or first radio link and thereafter establishing a new or second radio link with a second radio access unit while the call is temporarily suspended. Although this method can be used for inter-cell and inter-system handover in asynchronous radio environments it has a severe drawback in that when the set up of the new radio link fails, in particular in dense traffic areas, the first or any other radio link of the first radio access unit cannot be longer available for resumption of the call because it can be occupied by another call already, for example.